Inflatable balloons are employed to dilate stenotic arteries in angioplasty, i.e., percutaneous transluminal coronary angioplasty (PTCA), to dilate stenotic cardiac valves in valvuloplasty, and to deliver and reconfigure stents and stent-grafts. To prevent late lumen loss and restenosis, a stent is carried on a balloon, positioned and expanded to remain in a dilated vessel. Unfortunately, currently available balloon systems for angioplasty and for delivery and expansion of stents often fail to properly deploy the stent to produce a uniform diameter and cross-sectional area along the length of the stent. This results from the very nature of such a cylindrical balloon which is made of a thin, flexible membrane and hence can expand radially to different diameters along its longitudinal dimension. If an obstructive lesion in a blood vessel is of a firmer and less compliant tissue than the normal vascular wall, e.g., fibrous or calcified matter, such tissue presents a much greater resistance than the normal vascular wall to dilatation as the balloon expands. As a balloon is expanded, such stenoses, narrowings, and obstructions impinge upon the expanding balloon to cause an area of relative narrowing or distortion, a so-called “waist”. Correspondingly, a stent delivered on such a balloon will suffer from a similar distortion or “waist configuration” as it is conformably expanded with the expanding delivery balloon.
If the stent adopts a “waist configuration”, it is generally indicative of inadequate dilatation of the stenotic lesion within the blood vessel. The conventional approach to attempt to remedy this inadequacy has been to increase pressure within the balloon to expand the narrowed area. At times, a separate, higher pressure balloon is utilized. To produce such higher pressure balloons, flexible balloon membranes, which could rupture under increasing internal pressure, were replaced by more rigid balloon materials that permitted much higher internal pressures. However, the more rigid material still has an elastic limit. Balloons made of such material and excessively inflated will be permanently deformed, and may eventually burst. Such deformed balloons are much more difficult to remove from the patient. Moreover, increasing the pressure in the balloon increases the potential for rupture and serious harm to the patient.
Even the use of higher pressures permitted by more rigid balloons are insufficient to dilate some arterial stenoses, particularly if they have an annular configuration. With the stent not completely open and the lumen not fully dilated, the final therapeutic result is less than optimum. Some residual stenoses and a non-uniform cross-sectional area along the length of the stent will result.
Another problem frequently encountered during balloon dilation of stents is the occurrence of flaring of the stent at its ends. Stents are generally manufactured as an independent cylindrical structure or integrated in a sleeve that is slipped onto the balloon and adhered. Conventionally, the balloons are at least slightly longer than the stents that they carry.
As the balloon is inflated and it expands the stent, it typically meets less resistance at the ends of the stent and outside the confines of the stent than it does within the stent. Hence, at full inflation the balloon has a tendency to expand to a slightly larger diameter beyond the ends of the cylindrical stent than at the middle of the stent. The resulting different diameters of expansion are transferred to the stent causing a “trumpet-like” outward flaring of the stent at its ends, which is undesirable.
Additionally, there are several problems with the conventional methodology for delivering a stent to an occlusive site with a conventional balloon catheter. In trying to force the stent inside a tight occlusion, calcified matter or other irregularity at the occlusion often provides resistance against the leading edge of the balloon catheter and may resist entry and positioning of the stent. As the operator tries to advance or withdraw the catheter, the calcified matter or other irregularity may catch hold of the stent and capture it in place while the catheter is moved, causing the stem to separate from the balloon. If the stent is displaced with respect to the balloon, slips partially off the balloon, or separates from the balloon entirely, then the stent will not deploy properly. If the undeployed stent separates from the balloon entirely, retrieving the undeployed stent becomes a very serious problem. Similarly, a stent that is only partially deployed or is incorrectly positioned presents a very significant risk to the patient.
Where an occlusive site is only partially accessible by means of a conventional balloon catheter, i.e., only one end of an occlusion has an inner diameter that is of sufficient size to receive a balloon catheter, expansion of the balloon often causes the entire device to be squeezed and slip out of the occlusive site entirely. Thus, attempts to open such partially accessible occlusive sites often fail. To keep the prior art device within the occlusive site a great deal of force may need to be applied by the operator to prevent the device from slipping out. Such force causes additional stress at the occlusive site and in surrounding structures which present a further risk of rupturing the target site or causing damage to the surrounding areas. This same behavior is also observed in situations where an occlusion is irregular in diameter and the expanded balloon simply slips out of the occlusive site upon reaching a particular state of expansion.
The same difficulties are encountered in attempting to position and deploy a stent-graft with a conventional balloon catheter. Incomplete stent-graft deployment can result in troublesome endoleaks caused by inadequate or otherwise ineffective sealing of the stent-graft at the ends with the interior of the vessel. Such endoleaks allow a channel of blood flow to develop that bypasses the stent-graft, greatly reducing its effectiveness and potentially causing the stent-graft to migrate. Where stent-grafts are deployed to exclude aneurysms, such as in the endovascular repair of an abdominal aortic aneurysm, endoleaks are a very significant problem in that they may allow flow to an aneurysm that could cause the aneurysm to rupture.
U.S. Pat. No. 4,796,629, by one of the present inventors, describes a stiffened dilation balloon which addresses some of these problems by providing an expandable balloon which exerts greater expansion force on localized regions within the lumen. It has been found that the uniform expansion provided by the balloon catheter device described in U.S. Pat. No. 4,796,629 achieves superior results in dilating occluded vessels. As such a stiffened dilation balloon is expanded within an occlusion, the longitudinal stiffener, acting as a rigid beam, transmits expansion force applied to the entire length of the stiffener by the balloon to the localized points of resistance in the vessel. Thus, as compared to a conventional balloon, the force of dilation applied locally is considerably increased and dilatation of highly resistant lesions is greatly facilitated. Such a stiffened dilation balloon is capable of achieving the same degree of dilatation as a standard balloon delivery system but at lower pressures. The stiffeners significantly increase the rigidity of the balloon, reducing variations in the cross-sectional area of the balloon along its length and reducing the occurrence of annular regions with a narrowed waist.
Additionally, it has also been found that an inherent limitation of the balloon catheter device described in U.S. Pat. No. 4,796,629 is its limited maneuverability for deployment in the vascular system. The stiffeners of such a device are fairly rigid, have a fixed length in excess of the turning radius needed to navigate certain pathways and have a fixed, although not necessarily uniform, cross-section. As a result, it can be particularly difficult to navigate the device of U.S. Pat. No. 4,796,629 through the vascular system to reach certain occlusions in smaller vessels without utilizing non-standard entry locations or invasive manual procedures for straightening tortuous vascular pathways.